Dip process for coloring cadmium



Patented Oct. 3., 1950 signor to United Chromium, Incorporated, New

York, N; Y., a corporation of Delaware No Drawing. Application August10, 1948, Serial No. 43,532

2. Claims. (c1. 14s s.2)

This invention'relates to dip processes for coloring cadmium, andprovides improvements there- 1n. I I

This applicationis a continuation-in-part of my application Serial No.588,423, filed April 14, 1945, now Patent 2,483,510, October 4, 1949.

Black, adherent, corrosion resistant, surface conversion coatings oncadmium, obtainable by dipping treatment are greatly desired byindustry. Coatings heretofore produced by dipping methods have generallylacked adherence, do not have good corrosion resistance and do not haveas deep and lively a black color as desired. 7

The present invention provides a novel process forproducing deep toneblackasurface conversion coatings on cadmium, having good luster,superior corrosion resistance and good adherence. The coatings do notpeel or flake when the metal on which the coating is applied is sharplybent, and will withstand piercing and forming operations. The inventionprovides the only known process for directly producing black surfaceconversion coatings on cadmium which have good corrosion resistance. Theinvention further provides a process which acts with good speed toproduce the surface conversion coating.

According to the present invention the dip solution and composition usedin the process consists essentially of a chromate radical fromhexavalent chromium compounds, an auxiliary or activating acid radicaland silver ion. Other compounds may be included in the solution andcomposition for special purposes. 1

Various chromate solutions for forming surface conversion coatings oncadmium are known, and tests With numbers of these solutions to which asoluble silver salt was added showed that the benefits of the presentinvention are obtained. A common characteristic of these prior artchromate dip solutions is that they contain hexavalent chromiumradicals, obtained from dissolving chromates, dichromates and chromicacid, and mixtures thereof, and an auxiliary or activating acid radical.

Chloride radicals should not be used as an auxiliary acid radical in thepresent invention, and the presence of chlorides should be excluded orheld to a very small amount, as it forms a very insoluble compound withsilver and the nitrate radical except in small amounts is undesirable.

Silver ion may be obtained by dissolving any of the silver compoundsexcept the most insoluble. In the presence of a large amount ofchromate, silver will form silver chromate, which has limitedsolubility, on the order of a few tenths of a gram per liter. Ordinarilysilver nitrate is used as being the most available, but many othercompounds of silver may also be used, such as, for example,

the acetate, sulphate, carbonate, citrate, silicofluoride, tartrate,tungstate, and oxide. The silver compound is usually added in an amountin excess of that corresponding to the amount of silver ion which willremain in the chromate solution; in this way precipitation by smallamounts of chlorides in the solution will be compensated and also anautomatic source for replenishment of silver ions lost in the process isprovided.

{According to my view the silver ion in the dip solution increases thetendency of the cadmium to go into solution, i; e., to react with thechromate ions of the solution and thereby form the surface conversioncoating. Further, according to my view, the surface metal, in thepresence of thedip solution, acts to reduce some of the hexavalentchromium to trivalent chromium, and

these different constituents react on one another to form on the surfaceof the metal, black basic chromium chromate (Cr(0H) 2) 2CrO4.

The composition used in the process, of the present invention consistsessentially of the ingredients making up any known chromate dip solutionfor forming surface conversion coatings on cadmium with the addition ofa relatively small amount of a soluble silver compound.

The preferred composition consists essentially of a water solublehexavalent chromium compound (as chromic acid, or a soluble chromate ordichromate), and a compound which dissolves in water to yield anauxiliary or activating acid radical (as a sulphate, or a fluoride) withthe addition of a relatively small amount of water soluble silvercompound.

An example of the preferred composition, giving the optimum and rangesof the ingredients, follows:

EXAMPLE I Per Cent by Weight fifgg gfi -Optimum Range Optimum Range 16.58-24 30 15- 45 42. 7 20-55 78 40-100 0.3 0.020.7 few' tenths fewtenths-IL all.

The pH range is 1.0 to 3.0 (optimum 1.4 to 2.5)

Typical make-up ingredients (per cent by weight) Optimum Range EXAMPLEII Concentration in Solulcr Cent b3 Weight mm,

Optimum Range Optimum Range 16. 8-24 30 15- 45 F 42. 7 20-55 78* 1 40100Ag+ 0.3 0. 02-0. 7 few tenths few tenths g./.l g./l.

Typical make-up ingredients (per cent by weight) Optimum Range The clipis usually made by adding water to 1 to 3 pounds of the aforesaidcompositions to make a gallon of solution. 7

Temperature has an effect on the potential of the metal on which thesurface conversion coating is being formed, in contact with thesolution, increasing temperature making the metal more active. Goodresults are obtained with cold or warm solutions.

Articles are immersed until the coating has developed over the surfacethereof; ordinarily the time of immersion is from /2 to 5 minutes.

After immersion, the coated articles are rinsed in water and dried in asuitable manner, as by hot water, warm air, or centrifuging.

What is claimed is:

l. A method of forming black surface conversion coatings on cadmiumconsisting in dipping the cadmium in a water solution consistingessentially of a hexavalent chromium compound of the group consisting ofchromic acid, an alkali-metal chromate, and an alkali-metal dichromate,a water soluble acid or salt containing an activating or auxiliary acidradical of the group consisting of sulphate radicals and fluorideradicals, and a water soluble silver compound, the amounts by weightbeing: Cr of the hexavalent chromium compound -15 to g./l., activatingacid radical of the compound containing it 40 to g./1., and silver ofthe silver compoundfew tenths of a g./1., and then removing the cadmiumfrom the solution.

2. A method of forming black surface conversion coatings on cadmium,according to claim 1, wherein the solution has a pH from 1.0 to 3.

3. A method of forming black surface conversion coatings on cadmium,according to claim 1, wherein the solution has a pH from 1.4 to 2.5.

JESSE EDWIN STARECK.

REFERENCES CITED The following references are of record in the fileof'this patent:

UNITED STATES PATENTS Number Name Date 1,782,092 Gray et a1 Nov. 18,1930 2,106,904 Wilhelm Feb. 1, 1938 2,288,007 Lum June 30, 19422,483,510 Stareck Oct. 4, 1949

1. A METHOD OF FORMING BLACK SURFACE CONVERSION COATINGS ON CADMIUMCONSISTING IN DIPPING THE CADMIUM IN A WATER SOLUTION CONSISTINGESSENTIALLY OF A HEXAVALENT CHROMIUM COMPOUND OF THE GROUP CONSISTING OFCHROMIC ACID, AN ALKALI-METAL CHROMATE, AND AN ALKALI-METAL DICHROMATE,A WATER SOLUBLE ACID OR SALT CONTAINING AN ACTIVATING OR AUXILIARY ACIDRADICAL OF THE GOUP CONSISTING OF SULPHATE RADICALS AND FLUORIDERADICALS, AND A WATER SOLUBLE SILVER COMPOUND, THE AMOUNTS BY WEIGHTBEING: CR**VI OF THE HEXAVALENT CHROMIUM COMPOUND -15 TO 45 G./L.,ACTIVATING ACID RADICAL OF THE COMPOUND CONTAINING IT -40 TO 100 G./1.,AND SILVER COMPOUND-FEW TENTHS OF A G./1., AND THEN REMOVING THE CADMIUMFROM THE SOLUTION.